Kinetic investigations of the enzymatic and transport reactions of the Ca2+ pump in sarcoplasmic reticulum (SR) labelled with fluoroscein isothiocyanate have uncovered new evidence for the existence of protein subunit interactions in the mechanism of active Ca2+ translocations. The results suggest that subunit conformational interactions are involved in recycling of the transport site following Ca2+ transport. Three distinct passive Ca2+ efflux components have been resolved in passively-loaded SR membrane vesicles isolated from rat myocardium. The absence of an age difference in Ca2+ efflux suggests that the age- related decline in active Ca2+ transport is not due to an increased passive Ca2+ leak. Determination of the (Na+) dependence of the Na+-H+ exchanger over an extended range of concentrations has revealed a new class of high affinity Na+ binding sites that participate in Na+ translocation. The kinetic features of the Na+- H+ exchanger are consistent with a tetrameric subunit model in which the sites become sequentially available to transport Na+ after the initial turnover. Two Na+i-independent Ca2+ translocation pathways have been identified in cardiac sarcolemmnal vesicles that exhibit differential sensitivity to inorganic Ca2+ channel blockers. The pharmacologic and kinetic behavior associated with these pathways suggests that they might be involved in mediation of the transient and slow inward Ca2+ currents in cardiac muscle.